This invention relates to a wavelength modulation derivative spectrometer (WAMOS) and, more particularly, to a WAMOS that utilizes a wavelength modulation device to judge the completion of a process such as plasma etching in the manufacture of, for example, IC elements.
In the process of manufacturing semiconductor IC elements, use is made, as a means of detecting completion of such processes as plasma etching, plasma ashing or reactive ion etching, of a means that detects a variation in the plasma spectrum of the substance being processed. Where an aluminum wiring pattern for, for example, IC elements is formed by plasma etching, measurement is made of the plasma spectrum emitted from the substance during the etching process. FIG. 1A is a spectrum (direct spectrum) diagram based on the results of a conducted measurement. A prominent emission line spectrum, as exemplified by A (308.2 nm) in FIG. 1A, is a typical find among the spectral characteristics measured. With emission line spectrum A as a target spectrum, spectrometer detection wavelength .lambda. is fixed to center wavelength .lambda..sub.0 of the emission line spectrum to permit the intensity of emission line spectrum A to be measured continuously. The etching process is terminated when emission line spectrum A varies, as shown in FIG. 1B, greatly, this being regarded as an etching completion time.
Generally, the plasma spectrum characteristics normally contains spectra having various wavelengths emitted from substances other than the target substance. That is, disturbed spectra deriving from these substances overlaps the target spectrum, thus there being no possibility that correct measurement will be made of the intensity of the target spectrum. A broader spectrum and a linear pattern of emission line spectrum are mingled in the disturbed spectrum. It is therefore necessary that, in a range between the etching process time and the etching completion time, outstanding emission line spectra of greater intensity be found from among the whole range of spectral characteristics.
Upon comparison between the spectral characteristic at the etching process time in FIG. 1A and that at the etching completion time at FIG. 1B an emission line spectrum B is found in the neighborhood of 400.0 nm. As a result, the etching process time can be detected by matching the detection wavelength .lambda. of the spectrometer to the center wavelength .lambda..sub.0 of the emission line spectrum B and detecting a variation in the intensity of the spectrum B.
In the aforementioned continuous measurement of the variation in the intensity of the emission line spectrum B with the detection wavelength .lambda. of the spectrometer fixed at .lambda..sub.0 to detect the plasma etching completion time, however, the following drawbacks are encountered.
That is, the emission line spectrum B at the wavelength .lambda..sub.0 is often found to overlap a broader range of background spectrum.
With the full intensity of a whole emission line spectrum, that of the background spectrum and that of a variation, projected from the background spectrum, represented by c, b and a, respectively, it is necessary to measure a variation in the intensity a finally so that the completion of etching may be detected. In actual practice, the full intensity c of the emission line spectrum containing the background spectrum intensity b is measured by the spectrometer.
In general, the accuracy with which the intensity of the spectrum is measured by the spectrometer is on the order of 0.1% at most. Consequently, with the intensity a of the detection spectrum set smaller than the intensity b of the background spectrum, it has not been possible in this field of art to precisely measure such a variation intensity a.
In the plasma etching process of semiconductor IC elements, when the value of the intensity a ceases to be detected as shown in FIG. 1B, this aspect is judged as being an etching process completion time. If in this case the value of the intensity a cannot be accurately measured as set forth above, the intensity a is judged as reaching zero, in spite of the fact that the etching step has not yet been completed, sometimes resulting in the interruption of the etching process. Since in this case some area of the semiconductor surface is left unetched, there is a possibility that a lower yield will result in the manufacture of semiconductor IC elements.